The present invention is directed to an improved process and apparatus for producing oxygen-15 (.sup.15 O)-labeled butanol with sufficient product yield and purity for position emission tomography. In addition, the present invention is directed to the purified .sup.15 O-labeled butanol produced by the process of the invention, as well as to the use of the purified .sup.15 O-labeled butanol for the clinical diagnosis.
Positron emission tomography (PET) is a procedure that produces transverse sectional images of the body by the demonstration of the internal distribution of positron-emitting radionuclides. Along these lines, positron emission tomography uses measurements of the back-to-back emission of gamma rays from the annihilation of positrons emitted by radioactive tracers to map the distribution of these tracers in the human body.
One important function of positron emission tomography is the non-invasive assessment of regional cerebral blood flow (CBF) through the use of a number of well-known methods which rely on mathematic modeling of the distribution and/or kinetics of a particular highly retained or freely diffusible tracer (Berridge, M. S., Franceschini, M. P., Tewson, T. J., and Gould, K. L., Preparation of oxygen-15 butanol for positron tomography, J. Nucl. Med. 27, pp. 834-839, 1986). In this regard, a number of tracers have been developed for positron emission tomography. For example, oxygen-15(.sup.15 O)-labeled water is fairly commonly used for cerebral blood flow measurements. However, while .sup.15 O labeled water has proved to be useful, it reportedly leads to an underestimation of cerebral blood flow in areas of high flow rate because of a limitation of water extraction in the brain tissue (Murakami, M., Hagami, E., Sasaki, H., et al., Radiosynthesis of .sup.15 O labeled butanol for clinical use, 6th Int. Sym. Radiopharm. Chem., 81, 1986).
Furthermore, because butanol exhibits a greater degree of lipid solubility which results in the near complete free exchange of butanol into brain tissue, butanol, as opposed to water, has been recommended as being more ideally suited for use in measuring cerebral blood flow (Raichle, M. E., Martin, W. R. W., Herscovitch, P., Kilbourn, M. R., and Welch, M. J., J. Nucl. Med., 24, P63, 1983). Along this line, labeled butanol has now been shown to be a more accurate region cerebral blood flow tracer than labeled water and lower alcohols by studies of animal and human subjects. These studies, performed with radiocarbon labels such as .sup.11 C (Hack, S. N., Bergmann, S. R., Eichling, J. O., et al., Quantification of region myocardial perfusion by exponential infusion of [C-11]-butanol, IEEE Trans. Biomed. Eng., 30, pp. 716-722, 1983; Knapp, W. H., Helus, F., Oberdorfer, et al., .sup.11 C-Butanol for imaging of the blood flow distribution in tumor bearing animals, Eur. J. Nucl. Med., 10, pp. 540-548, 1985; Van Uitert, R. L., Sage, J. I., Levy, D. E., and Duffy, T. E., Comparison of radio-labeled butanol and iodantipyrine as cerebral blood flow markers, Brain Research, 222, pp. 365-372, 1981; Herscovitch, P., Raichle, M. E., Kilbourn, M. R., Welch, M. J., Positron emission tomography measurement of cerebral blood flow and permeability - surface area product of water using [.sup.15 O] water and [.sup.11 O] butanol, J. Cerebral Blood Flow and Metabolism, 7, pp. 527-542, 1987; Raichle, M. E., Eichling, J. O., Straatman, M. G., Welch, M. J., Larson, K. B., and Ter-Pogossian, M. M., Blood-brain barrier permeability of .sup.11 C labeled alcohols and 15O labeled water, Am. J. of Physiology, 230, pp. 543-552, 1976; Kothari, P. J., Finn, R. D., Vora, M. M., Boothe, T. E., Emran, A. M., and Kabalka, G. W., 1-[ .sup.11 C]Butanol: synthesis and development as a radiopharmaceutical for blood flow measurements, Int. J. Appl. Radiat. Isot., 36, pp. 412-413, 1985; Raiche, M. E., Eichling, J. O., Straatman, M. G., Welch, M. J., Larson, K. B., Ter-Pogossian, M. M., Blood-brain barrier permeability of .sup.11 C-labeled alcohols and .sup.15 O-labeled water, Am. J. Physiol., Vol. 230, No. 2, 1976) and .sup.14 C labeled butanol (Ginsberg, M. D., Busto, R., and Harik S. I., Regional blood-brain barrier permeability to water and cerebral blood flow during status epilepticus: insensitivity to norepinephrine depletion, Brain Research, 337, pp. 59-71, 1985), show that butanol is freely permeable in the brain, due to its greater lipid solubility. Thus, the technique of using butanol for obtaining blood flow information has been clearly validated using .sup.11 C and .sup.14 C labeled butanol.
Notwithstanding the above, a very short-lived tracer is required for routine, sequential clinical positron emission tomographic studies. The long half-life of .sup.11 C and .sup.14 C (20 minutes) is inconvenient in common medical imaging procedures that use several sequential measurements of flow and measurements involving other radiopharmaceuticals. However, the short two minute half-life of .sup.15 O allows for repeat studies at ten minute intervals and also results in a low radiation dose to the patient. Therefore, it is now recognized that several sequential studies of one subject can be performed with improved accuracy if .sup.15 O-butanol is utilized as the tracer material.
Nevertheless, a problem that has slowed to some degree the use of butanol or any other alcohol labeled with .sup.15 O is that the processes available for the synthetic incorporation of .sup.15 O have not been sufficiently rapid or efficient. Along these lines, the two minute half-life of .sup.15 O has inhibited the overall production and use of .sup.15 O labeled alcohols as a tracer for positron emission tomography.
An additional restraint is that the tracer, once synthesized must be amenable to rapid purification. Along this line, the synthesized .sup.15 O-labeled butanol cannot be used for human subjects if it is not pure. .sup.15 O-labeled butanol is useless for quantitative studies if it contains the labeled by-product, .sup.15 O-labeled water. However, the two minute half-life of .sup.15 O limits the time available for purification. A very rapid but reliable purification process is necessary in order to produce useful quantities of good quality product.
In an attempt to overcome the above-noted difficulties, and to produce .sup.15 O butanol in an amount and quality sufficient for positron emission tomography use, the applicant previously described (Berridge, M. S., Franceschini, M. P., Tewson, T. J., and Gould, K. L., Preparation of oxygen-15 butanol for positron tomography, J. Nucl. Med. 27, pp. 834-839, 1986) a labeling procedure based on the work of Kabalka (Kabalka, G. W., Incorporation of stable and radioactive isotopes via organoborane chemistry. Accts. Chem. Res. 17, pp. 215-221, 1984; Kabalka, G. W. Lambrecht, R. M., Sajjad, M., Fowler, J. S., Wolf, A. P., Kunda, S. A., McCollum, G. W., and MacGregor, R., Synthesis of .sup.15 O labeled butanol via organoborane chemistry, 5th International Symposium on Radiopharmaceutical Chemistry, Tokyo, Japan, IV-P-24:265, 1985) with a tri-n-butylborane. Although the process described by Kabalka, et al. did not achieve a sufficiently high yield or purity in general use to allow for clinical applications, the basic reaction disclosed by Kabalka, et al. was utilized in applicant's process.
More particularly, in applicant's previous study, butanol was labeled with .sup.15 O using the reaction of tri-n-butyl borane with oxygen gas in a glass vessel followed by C-18 cartridge purification. The preparation of .sup.15 O-butanol was performed using Kabalka's published reaction: ##STR1##
Furthermore, the .sup.14 N(d,n).sup.15 O reaction on nitrogen gas containing 1-2% oxygen was used in applicant's previous process for the production of .sup.15 O-labeled oxygen. The .sup.15 O labeled oxygen was then reacted with tri-n-butyl borane by bubbling the O.sub.2 through the tri-n-butylborane in a glass reaction vessel to produce a boron-complex, which was subsequently hydrolyzed by the addition of water. The solution was then purified through C-18 cartridge purification. A 50% conversion of .sup.15 O to butanol was achieved through the use of this process with the remaining 50% comprised of labeled water. Pure butanol was recovered in about 10% radiochemical yield or 40% chemical yield with a synthesis time of four minutes.
However, while applicant's previous process was effective in producing .sup.15 O-labeled butanol, the production of butanol (i.e. 50 mCi) was too low for reliable clinical use. In addition, the production time for synthesis and purification, i.e. four minutes, was only marginal at best for positron emission tomography.
Subsequently to applicant's process, Ido and associates reported (Murakami, M., Hagami, E., Sasaki, H., Kondo, Y., Mizusawa, S., Nakamichi, H., Iida, H., Miura, S., Kanno, I., Vemura, K., and Ido, T., Radiosynthesis of .sup.15 O labeled butanol for clinical use. 6th International Symposium Radiopharm. Chem. 81, 1986) the use of a Sep-Pak C-18 cartridge (Waters Chromatography Div., Millipore Corp., Milford, Mass.) for both reagent support and purification with similar results. The Ido, et al. procedure was substantially similar to applicant's previous procedure except that [.sup.15 O] oxygen was trapped on the surface of the packed agent (an 18 carbon silane) in the micro column of a Sep-Pak C-18 cartridge, and a lower percentage of oxygen was present in the target gas (i.e. 0.5% O.sub.2 in N.sub.2). Specifically, the [.sup.15 O]O.sub.2 was sent by helium gas flow to a Sep-Pak C-18 silane or silica cartridge which contained solvent-free tributyl borane (1 mmole). After the trapping of [.sup.15 O]O.sub.2 (0.2 mmol), another Sep-Pak C-18 (C-18 carbone silane) cartridge was connected to the outlet. The 1.5 ml of water was passed through the two columns to hydrolyze the tributyl borane-O.sub.2 complex. At this step, the radioactive impurity was eluted, and the unreacted tributylborane was retained in the cartridges. Subsequently, a strong amine exchange resin column (OH.sup.- type, 0.2 ml) was connected under the two Sep-Pak cartridges, and .sup.15 O-butanol was eluted by the addition of water to the three columns. At this step, the non-radioactive impurity was absorbed to the resin column. The eluate containing the .sup.15 O butanol was then collected for clinical use. However, while Ido, et al. reported higher yields and purities, their results were difficult to reproduce. In addition, the product purity and/or yield of the Ido, et al. process may be insufficient for clinical use.
Applicant's present invention is directed to a new and improved method for synthesizing .sup.15 O-labeled butanol which overcomes the purity and yield restrictions produced by the prior art. In addition, in the investigation which produced the present invention, the applicant examined the effects of several parameters on the overall synthesis process for producing .sup.15 O-labeled butanol. The overall result is a new process and apparatus for producing .sup.15 O-labeled butanol which have sufficient product yield and purity for use in positron emission tomography.